# Biology of Gender Identity
- The Endocrine Society states there are biological underpinnings of gender identity. [^EndocrineSociety2020]
1. attempts to change gender identity in intersex patients to match external genitalia or chromosomes are typically unsuccessful;
2. identical twins (who share the exact same genetic background) are more likely to both experience transgender identity as compared to fraternal (non-identical) twins;
3. among individuals with female chromosomes (XX), rates of male gender identity are higher for those exposed to higher levels of androgens _in utero_ relative to those without such exposure, and male (XY)-chromosome individuals with complete androgen insensitivity syndrome typically have female gender identity;
4. there are associations of certain brain scan or staining patterns with gender identity rather than external genitalia or chromosomes.
# Genetics / Heritability of Trans Identities
- "The fact that trans identities are at least partially genetic is not really disputable at this point." [^DocImpossible2023]
- "Twin studies for trans identities have confirmed strong genetic roots in [over a dozen studies](https://www.nature.com/articles/s41598-022-17749-0/tables/3) now. While results vary from study to study, due largely to methodological differences, most studies find that between 30-50% of gender incongruence is heritable." [^DocImpossible2023]
# Brain Science
- What physiologically changes _make_ us trans is a much more fraught question. The candidate most often cited—sexually dimorphic brain shape—was decisively struck down in 2021 by [a massive metastudy](https://www.sciencedirect.com/science/article/pii/S0149763421000804#:~:text=Task%2Dbased%20fMRI%20has%20especially,is%20not%20%E2%80%9Csexually%20dimorphic.%E2%80%9D) that shredded the very idea that brains are shaped differently based on sex. [^DocImpossible2023]
# Intersex
- The rate of sexually ambiguous traits may be as high as 2% of live births. [^Blackless-FaustoSterling-EtAl-2000]
[^DocImpossible2023]: Doc Impossible. 2023. “There Are 385.5 Million Trans People Alive Today.” Substack.com. Stained Glass Woman. March 13, 2023. https://stainedglasswoman.substack.com/p/there-are-3855-million-trans-people.
[^Blackless-FaustoSterling-EtAl-2000]: Blackless, Melanie, Anthony Charuvastra, Amanda Derryck, Anne Fausto-Sterling, Karl Lauzanne, and Ellen Lee. “How Sexually Dimorphic Are We? Review and Synthesis.” _American Journal of Human Biology_ 12, no. 2 (2000): 151–66. https://doi.org/10.1002/(SICI)1520-6300(200003/04)12:2<151::AID-AJHB1>3.0.CO;2-F.
[^EndocrineSociety2020]: Endocrine Society. 2020. “Transgender Health: An Endocrine Society Position Statement.” Endocrine.org. Endocrine Society. December 16, 2020. https://www.endocrine.org/advocacy/position-statements/transgender-health.